If the latest Ubuntu ran on PPC Macs, that'd be a marked improvement in their support. The last version to actually boot and install was 7.04. A better choice would be Debian, which still supports PPC.
"'Tis true, 'tis pity, and pity 'tis 'tis true." -- Poloniouius, in Willie the Shake's _Hamlet, Prince of Darkness_